Steam turbines of the above type comprise a high-pressure module, usually a medium-pressure module, and at least one low-pressure module. In some installations there may be several low pressure modules. High pressure, high temperature steam from the boiler is routed to the high-pressure turbine module and then in turn to the medium-pressure and low-pressure turbine modules as the temperature and pressure of the steam is reduced due to extraction of energy by the turbines. Finally, the steam exhausts from the, or each, low pressure turbine module into a condenser.
In the specific arrangements disclosed herein, the or each low-pressure turbine module has an external casing, called the exhaust hood, and an internal turbine casing from which steam exhausts into the exhaust hood. Steam then passes from the exhaust hood into a condenser, which is located below the exhaust hood. The turbine casing is an assembly comprising (a) a generally cylindrical casing portion that contains axial flow, low pressure steam turbine stages, and (b) a diffusing exhaust duct structure arranged to turn the steam flow away from its axial path at the exit of the turbine and exhaust it radially outwards into the exhaust hood. The exhaust hood encloses the turbine casing, but does not support it, whereas the walls of the exhaust hood and the condenser are joined rigidly together and hence form an assembly distinct from the turbine casing. During operation of the turbine, the pressure of the steam in the condenser and at the exit of the low-pressure turbine is only of the order of a few tens of millibars, so the exhaust duct structure, the exhaust hood and the condenser must together form a steam- and air-tight enclosure, which may be considered as a vacuum enclosure.
The steam turbine has a turbine rotor shaft, which penetrates the exhaust duct structure in order to transmit mechanical power to the electrical generator outside the turbine module. The rotor shaft is supported in bearing assemblies that are integrated into the exhaust duct structure of the turbine casing through bearing supports, which in turn are connected to and supported by a massive concrete foundation block. To limit the loading on the foundation block, it is arranged that it bears only the weight of the turbine casing and its contents, the combined weight of the turbine and the foundation block being borne by a civil engineering structure underneath the foundation block. The combined weight of the exhaust hood and condenser is transmitted to a base plate of the civil engineering structure independently of the load path that transmits the combined weight of the turbine and the foundation block.
To facilitate assembly and disassembly of the turbine module, the exhaust hood, the turbine casing (including the exhaust duct structure) and the bearing assemblies each comprise a lower part and an upper part, which are bolted and sealed together along bolting flanges at a common plane.
A number of problems must be addressed simultaneously in the design of an efficient vacuum seal for the turbine module.                (a) Structural isolation of the exhaust hood from the turbine casing, which is consequent on the weights of the turbine and the exhaust hood being supported independently of each other, must not be compromised by the vacuum seal.        (b) Dynamic isolation of the exhaust hood from the turbine casing must be maintained during operation, particularly during start-up and run-down of the turbine, against thermally induced axial and radial movements (growth and shrinkage) of the turbine casing relative to the exhaust hood. Additionally, there may be transient movements of the turbine casing as the turbine adjusts to onset and removal of steam loading on the turbine rotor blades and diaphragms and changes of clearances during thermal growth and shrinkage. Furthermore, the exhaust hood/condenser enclosure will also experience thermally induced growth and shrinkage at different rates and amounts from the turbine casing and rotor, Hence, the vacuum seal must not interfere with the dynamic isolation of the exhaust hood and turbine casing.        (c) For operational and maintenance purposes the alignment of the turbine rotor shaft should be adjustable without the need to enter the turbine module enclosure and without interfering with the integrity of the vacuum seal.        